Add some documentation

[mardy]

Hi! I was about to start writing a Qt port of python's html.parser module, when I found this project. It looks exactly like what I need, except that it's not clear how to use it :-)

The example command provided does not show how to hook up to the various processing phases, and the main header file htmlparser.h does not offer any virtual methods that could be reimplemented in order to be notified of the various elements being parsed.